Projection with curved speckle reduction element surface

ABSTRACT

In a coherent light projection system including an image forming system, a relay system, a speckle reduction element, and a projection subsystem, the relay system can have a first f-number, and the projection subsystem can have a second f-number less than the first f-number. The relay system can have a first working distance, and the projection subsystem can have a second working distance less than the first working distance. The image forming system can project an initial image having a first size, and an intermediate image can have a second size greater than or equal to the first size. The speckle reduction element can have a curved surface through which the intermediate image is transferred. The speckle reduction element can include a lenslet arrangement formed on a surface thereof. The speckle reduction element can be moved in a direction parallel to an optical axis of the speckle reduction element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is filed concurrently with and has related subjectmatter to:

-   -   U.S. patent application Ser. No. ______, titled “Projection with        Slow Relay and Fast Projection Subsystems”, with Barry        Silverstein as the first named inventor;    -   U.S. patent application Ser. No. ______, titled “Projection with        Larger Intermediate Image”, with Joseph R. Bietry as the first        named inventor;    -   U.S. patent application Ser. No. ______, titled “Projection with        Lenslet Arrangement on Speckle Reduction Element”, with Barry        Silverstein as the first named inventor; and    -   U.S. patent application Ser. No. ______, titled “Out-of-Plane        Motion of Speckle Reduction Element”, with Barry Silverstein as        the first named inventor,    -   each of which is incorporated herein by reference in its        entirety.

FIELD OF THE INVENTION

The present invention relates generally to digital image projection andmore particularly to a coherent light projection system providingspeckle compensation.

BACKGROUND OF THE INVENTION

Conventional projection lenses used for projecting an image onto adisplay surface are designed with relatively fast optics. This isparticularly true for cinema projection, where traditional filmprojection lenses may be as fast as ˜f/10.8, and in the emergingtechnology of digital cinema, lenses are often ˜f/2.5. These low f/#values and correspondingly high angular light are due, in large part, tothe large etendue light sources that are used, such as various types ofvery bright arc lamps and similar light sources, along with the desireto utilize as much of this light as possible.

In the case of digital cinema projection, the image content is providedvia pixelated spatial light modulators, such as LCD and LCOS modulators,Digital Micromirror Devices (DMDs), and in particular, the DLP (DigitalLight Processor) from Texas Instruments, Inc., Dallas, Tex. Individualpixels of these electronic light modulation devices are modulated on apixel-addressed basis to impart image data to a transiting light beam.To enable cinema projection, large versions of the devices, with activeareas of ˜400-600 mm² are used, to be compatible and light efficientwhen used with the large-etendue xenon lamp light sources used forcinema projection. However, we have determined that these large-etenduelight sources impact the projector design in various disadvantageousways, including size and cost of the optical components, thermal loadand stress on these components, and the optical imaging performance andimage quality provided by the optics. For example, the highly angularlight incident transiting the spatial light modulator device, and itsassociated polarization optics, unfavorably impact the projected imagequality, with peak contrast and contrast uniformity deficiencies.

In greater detail, the illumination and projection subsystems of digitalprojection systems are typically more complex than their equivalents intraditional film-based systems. In particular, in the digital systems,the projection lens systems are often burdened with different andadditional requirements compared to the conventional projection optics.As one example, the projection lenses for the digital systems aretypically required to provide a long back focal length or workingdistance, that is, the distance between the last lens surface and thespatial light modulator. Working distances in excess of 2 times the lensfocal length are needed in most cases, in order to accommodate a numberof optical components used to combine modulated light from the differentcolor paths onto a common optical axis and, depending on the type ofspatial light modulator used, to provide polarization, filtering, andother conditioning and guiding of the light. Taken together, the longback focal length and speed requirements (low F#) combine to drivecomplex lens designs using large elements, as can be well appreciated bythose skilled in the optical design arts. As a result, projection lensesused for large venue or digital cinema projection systems are quiteexpensive, particularly when compared to conventional projection lenses,such as those used in film-based projectors.

As one attempt to reduce this magnitude of this problem, a system, asdescribed in commonly assigned U.S. Pat. No. 6,808,269 entitled“Projection Apparatus Using Spatial Light Modulator” to Cobb, usesimaging relay lenses. Each modulator is imaged by a relay lens to createa real aerial magnified intermediate image near the exit face of acombiner prism. The large numerical aperture (NA) at the modulator planeis reduced, for example by two times, increasing the F# by acorresponding two times. The three-color images are combined through aprism, and then imaged by a common projection lens to the screen.Although the overall system, with the three imaging relays, is increasedin complexity, that increased complexity and cost is more thancompensated for by the simplicity of the projection lens, which works ata larger F#, without the working distance requirements.

As another approach, the use of visible lasers, having an advantageouslysmall etendue as compared with conventional light sources, offers anopportunity to provide simplified system optics, for example, byenabling projection lenses having similar levels of modest complexity asdo the lenses used for film-based projection. In recent years, visiblelaser light sources have improved in cost, complexity, and performance,thereby becoming more viable for use in projection, including forcinema. Lasers may provide a range of advantages for image projection,including an expanded color gamut, but their small etendue isparticularly advantageous for digital systems based on LCDs, DLP, andother types of light modulators, smaller, slower, and cheaper lenselements, with values in the f/8 range or slower may be used, whilestill providing light of sufficient visible flux for the cinemaapplication, as well as other projection applications. It is noted thatlasers also enable other modulator types to be used for projection, suchas the Grating Electromechanical (GEMS) modulators, which are lineararray devices that utilize diffraction to generate the image data, andwhich require a small etendue.

Lasers provide many potential substantial advantages for projectionsystems, including a greatly expanded color gamut, potentially long lifesources, and simplified optical designs. However, lasers also introducespeckle, which occurs as result of the coherent interference oflocalized reflections from the scattering surface of the display screen.Speckle is a high contrast granular noise source that significantlydegrades image quality. It is known in the imaging arts that speckle canbe reduced in a number of ways, such as by superimposing a number ofuncorrelated speckle patterns, or using variations in frequency orpolarization. Many of these methods are disclosed in “Speckle Phenomenain Optics: Theory and Applications” by Joseph W. Goodman. As one exampleof a speckle reduction method pertaining to projection, the displayscreen is rapidly moved with oscillating motion, generally following asmall circle or ellipse about the optical axis. As the screen moves,speckle changes, as localized interactions of the laser light withscattering features are altered by the screen motion. When thisoscillating motion is sufficiently fast, speckle visibility is reducedby temporal and spatial averaging, and speckle can become imperceptibleto the viewers. Yet another strategy for speckle reduction is to placean optical diffuser at an intermediate image plane internal to theprojector, and prior to the projection lens. Oscillation of the diffuserthen has the effect of reducing viewer perception of speckle.

A variety of optical diffusers have been used for laser projectionspeckle reduction, including ground glass, volume, holographic, andlenslet based devices. As one example, in the apparatus disclosed inU.S. Pat. No. 6,747,781 entitled “Method, Apparatus, and Diffuser forReducing Laser Speckle” to Trisnadi, which uses a diffuser patterned asa Hadamard matrix, in conjunction with a diffractive linear arraymodulator (GLV) to provide temporal phase variation to an intermediateimage of a scanned line of modulated light. This diffuser is constructedof an array of diffusing phase cells, each of which is subdivided into Ncell partitions, whose pattern is determined by the Hadamard matrixcalculations. An exemplary cell can be 24 μm square and comprise N=64cell partitions that are 3 μm square. The cell partitions either are anarea of the base surface, or a raised, mesa-like area, pi (π) high. Ifthe temporal phase variation provide by the diffuser motion and the cellpatterning are appropriate, phase variations in the transiting laserbeams are decorrelated, enabling speckle reduction. Specially designedprojection and scanning optics are then required in order to projecteach conditioned line of light onto the display screen. Typically, theprojection lens used for such a line-scanned device has an f/# of 2.5.While Trisnadi provides effective reduction of projected speckle,speckle reduction is only one aspect of the design of a laser projectionsystem. Speckle reduction provided by a moving diffuser located at aninternal intermediate image plane, that is then imaged to a screen,introduces various further problems, including a reduction image quality(resolution or MTF), light loss from diffusion (scatter or diffraction),and a requirement for faster imaging optics to collect diffused light.

Although many speckle reduction techniques, such as these, exist, thereis a continuing need in the art for improved techniques that reducespeckle perception for projected images, while also enabling advantageddesigns and system performance from laser projection systems.

SUMMARY

The above-described problems are addressed and a technical solution isachieved in the art by a system and a method for coherent lightprojection, according to various embodiments of the present invention.In embodiments of the present invention, a coherent light source systememits coherent light. An image forming system interacts with thecoherent light in a manner consistent with image data. A relay systemforms an intermediate image at an intermediate image plane from coherentlight output from the image forming system. The intermediate image is anaerial real image. A speckle reduction element is located at orsubstantially at the intermediate image plane. A movement generatingsystem moves the speckle reduction element, and

a projection subsystem projects the intermediate image, as transferredor passed through the speckle reduction element.

In some embodiments of the present invention, the relay system has afirst f-number, and the projection subsystem has a second f-number lessthan the first f-number. In some of these embodiments, the secondf-number is at least half the first f-number. For example, the firstf-number can be f/6 or greater, and the second f-number can be f 3 orsmaller.

In some embodiments of the present invention, the relay system has afirst working distance, and the projection subsystem has a secondworking distance less than the first working distance. In some of theseembodiments, the second working distance is at least half the firstworking distance. For example, the first working distance can be 100 mmor greater, and the second working distance can be 50 mm or smaller.

In some embodiments of the present invention, the image forming systemprojects an initial image having a first size, and the intermediateimage has a second size greater than or equal to the first size at theintermediate image plane. In some of these embodiments, the second sizeis consistent with a 16 mm, 35 mm, or 70 mm film format. Also in some ofthese embodiments, the projection subsystem corrects for film buckleeffects.

In some embodiments of the present invention, the speckle reductionelement has a curved surface through which the intermediate image istransferred. In some of these embodiments, the projection subsystemcorrects for film buckle effects, and the curved surface compensates forthe projection subsystem's correction of the film buckle effects. Thecurved surface can be a surface through which the intermediate image isreceived by the speckle reduction element or through which theintermediate image exits the speckle reduction element. The curvedsurface of the speckle reduction element can be an etched or polishedsurface, and it can include randomly or substantially randomlydistributed surface structures, such as lenslets.

In some embodiments of the present invention, the speckle reductionelement includes a lenslet arrangement formed on a surface of thespeckle reduction element, the lenslet arrangement including lensletseach having an aperture. Each of all or substantially all of the lensletapertures is greater than or equal to a size of a pixel of theintermediate image at the intermediate image plane. The lensletarrangement can have a random or substantially random distribution oflenslets. The lenslets can be in or substantially in a hexagonal,linear, or diagonal pattern. And, the lenslets can be abutting ornon-abutting. If they are non-abutting, a spacing between the lensletscan be or substantially be large enough to allow a diffusion from thespacing to pass into an acceptance aperture of the projection lens. Thespacing between the lenslets can be greater than or equal to a size of apixel of the intermediate image at the intermediate image plane. In someembodiments, the movement-generating system moves the speckle reductionelement in-plane a distance that is greater than or equal to a period oflenslet repetition. In some embodiments, an acceptance aperture of alens in the projection subsystem captures diffusion caused by spacingsbetween the lenslets or valleys between abutting lenslets. In someembodiments, an acceptance aperture of a lens in the projectionsubsystem captures a fourth order energy or below of the reduced-speckleimage. In some embodiments, the lenslet arrangement passes a fourthorder energy or lower of the reduced-speckle image into an acceptanceaperture of the projection subsystem.

In some embodiments of the present invention, the movement-generatingsystem causes motion of the speckle reduction element in a directionparallel to an optical axis of the speckle reduction element. In some ofthese embodiments, the motion is within a depth of focus of theprojection subsystem. Also in some of these embodiments, the motion iswithin a depth of focus of the relay system. The motion can furtherinclude motion in a direction perpendicular to the optical axis of thespeckle reduction element.

Various embodiments of the present invention are particularlywell-suited for spatial light modulators such as DLP devices thatmodulate light from a laser or other high brightness light source withcoherence. Various embodiments of the present invention provide anoptical system that allows the use of conventional type projection lenselements and takes advantage of high brightness that can be obtainedusing laser light.

These and other aspects, objects, features and advantages of the presentinvention will be more clearly understood and appreciated from a reviewof the following detailed description of the preferred embodiments andappended claims, and by reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more readily understood from the detaileddescription of exemplary embodiments presented below considered inconjunction with the attached drawings, of which:

FIG. 1 is a diagram showing some common components used in embodimentsof the present invention;

FIG. 2 is a diagram showing an apparatus of an embodiment of the presentinvention for digital image projection with reduced speckle;

FIG. 3 is a diagram showing an apparatus of an embodiment of the presentinvention with approximate f# and acceptance aperture relationships;

FIG. 4 is a diagram showing an apparatus of an embodiment of the presentinvention with a speckle reduction element having a curved surface;

FIGS. 5 a and 5 b illustrate internal system images, including anintermediate image and a diffused image, respectively;

FIG. 5 c illustrates film buckle and film imaging as occurs in aconventional film-based projection system;

FIG. 6 a is a diagram of an embodiment of a speckle reduction elementthat utilizes sparse microlenses; and

FIG. 6 b illustrates the use of a speckle reduction element inincreasing angular diversity of imaging through a projection lens,according to an embodiment of the present invention.

DETAILED DESCRIPTION

For the detailed description that follows, it is to be understood thatelements not specifically shown or described may take various forms wellknown to those skilled in the art. Figures shown and described hereinare provided to illustrate principles of operation and componentrelationships along their respective optical paths according toembodiments of the present invention and may not show actual size orscale. Some exaggeration may be necessary in order to emphasize basicstructural relationships or principles of operation. In some cases,components that normally lie in the optical path of the projectionapparatus are not shown, in order to describe the operation ofprojection optics more clearly.

The invention is inclusive of combinations of the embodiments describedherein. References to a particular embodiment and the like refer tofeatures that are present in at least one embodiment of the invention.Separate references to “an embodiment” or “particular embodiments” orthe like do not necessarily refer to the same embodiment or embodiments;however, such embodiments are not mutually exclusive, unless soindicated or as are readily apparent to one of skill in the art. The useof singular and/or plural in referring to the “method” or “methods” andthe like is not limiting.

The term “f-number” or f/# as used in the present disclosure has itsconventional meaning as the ratio of focal length to acceptance aperturediameter. Further, unless otherwise explicitly noted or required bycontext, the word “or” is used in this disclosure in a non-exclusivesense.

FIG. 1 illustrates a simplified schematic of several common coherentlight projection system components used in various embodiments of thepresent invention. Later figures and the following description introduceadditional components that are added to the common componentsillustrated in FIG. 1. In this regard, FIG. 1 shows a coherent lightprojection system 10 with a coherent light source system that emitshighly coherent light. In the case of FIG. 1, the coherent light systemincludes a coherent light source 16 r, 16 g, and 16 b for each of red,green, and blue color channels, respectively. However, other colorchannels may be used. Also in the case of FIG. 1, the coherent lightsources 16 r, 16 g, 16 b are laser light sources, such as directemission diode laser arrays, fiber lasers, or IR pumped, harmonicconversion lasers. However, any coherent or partially coherent lightsource with sufficient brightness and beam qualities can be used. Forexample, visible wavelength super luminescent diodes (SLEDs) may beused.

Light emitted from the coherent light sources 16 r, 16 g, 16 b isreceived by an image forming system, which, in the case of FIG. 1,includes spatial light modulators 12 r, 12 g, 12 b (such as DLP (digitalmicromirror) devices) and a combining element (such as a dichroiccombiner 14). Each light modulator 12 r, 12 g, 12 b lies at an objectplane 5 r, 5 g, 5 b, respectively, of a projection system, in this case,an imaging lens 20 a of a projection subsystem 20. In addition, eachspatial light modulator 12 r, 12 g, 12 b is image conjugate to adisplayed image plane 7, at display surface 30, where a screen can belocated. This arrangement can be used for an LCD or other type of lightmodulator.

During operation of the coherent light projection system 10, the lightmodulators 12 r, 12 g, 12 b interact with the coherent light emittedfrom the light sources 16 r, 16 g, 16 b, in a manner consistent withimage data, such as image data representing an image frame in a movie.In this regard, control signals are provided to the light modulators 12r, 12 g, 12 b by a data processing system (not shown), such as a controlsystem, that controls the light modulators 12 r, 12 g, 12 b in themanner consistent with image data using techniques and equipment knownin the art. In particular, the light modulators 12 comprisetwo-dimensional arrays of addressable modulator pixels (not shown) thatmodulate incident light in accordance with the image data signals. Lightmodulation can be provided by a variety of means, including redirectionby tilting of micro-mirrors (DLP), polarization rotation (LCOS or LCD),light scattering, absorption, or diffraction.

The modulated light from the light modulators 12 r, 12 g, 12 b iscombined onto the same optical path, axis O, at the dichroic combiner14. Light combined by the combiner 14 ultimately reaches the projectionsubsystem 20 including, in this case, an illustrative pair of lenses 20a, 20 b, which project images of the image content on the displaysurface 30.

Some of the problems that face the optical systems designer can bebetter appreciated by considering the simplified schematic diagram ofFIG. 1. In the system 10, a long working distance is needed, as lightfrom multiple light modulators 12 r, 12 g, 12 b is combined via combiner14 before being projected to the display surface 30 by the projectionsubsystem 20. Also, it can be advantageous to have projection subsystem20 operate at a large f# (such as f/6 or higher) in object space, sothat the long working distance is more readily and inexpensivelyachieved. Additionally, by not capturing the light at large angles, theprojection subsystem 20 would not pick up as much unwanted stray lightthat can be scattered from nearby surface structures. For example,components such as MEMS devices (such as the DLP modulators), lenselement edges and defects, and other structures can scatter light withinthe imaging system. This scattered light, or flare light, can passthrough the lens to the screen and reduce both wide area image contrast(ANSI contrast) and localized or image detail contrast, therebyaffecting the apparent screen blackness and resolvable detail. On theother hand, to reduce speckle visibility from a coherent sourceprojection subsystem 20, it would be preferable to deliver convergentlight to the display surface 30 with a large angular width (largenumerical aperture (“NA”)). However, assuming a constant magnificationfrom the projection subsystem 20, this means that the angular width ofthe light on the modulator side is also large, for example, having a lowf # (e.g., f/3 or lower). Thus, the desire to reduce potential specklevisibility is in conflict with the needs to reduce lens complexity andcost and minimize collection of internally scattered light.

The schematic block diagram of FIG. 2 shows a coherent light projectionsystem 50, according to an embodiment of the present invention thatalters the basic design of FIG. 1 in order to reduce speckle visibility.In particular, the light passing through combiner 14 is directed througha relay system, in this case, a relay lens 18. Relay lens 18, comprisingat least one relay lens element L1, is positioned to have each lightmodulator 12 r, 12 g, and 12 b as objects that are image conjugate to anintermediate image 22 formed at an intermediate image plane 21. Theintermediate image 22 is an aerial real image formed by the relay system(e.g., relay lens L1) and other upstream optics at the intermediateimage plane 21. An aerial real image is an image located in space thatcould be viewed if a screen or other display structure were placed atthe corresponding image plane, in this case, intermediate image plane21. In this instance, the aerial intermediate image 22 shown in FIG. 5 acomprises an array of intermediate image pixels 23 that are pixel imagesof the modulator pixels. In particular, the intermediate image pixels 23comprise overlapped and aligned images of corresponding pixel imagesfrom the red, green and blue modulators (12 r, 12 g, 12 b). Preferably,the corresponding imaged modulator pixels are co-aligned to within ¼pixel error or better across the entire intermediate image 22 atintermediate image plane 21. The intermediate image plane 21 is locatedwithin or substantially within a speckle reduction system, which, inthis case, comprises a speckle reduction element 40 and a movementgenerating system, in this case, an actuator 49, that moves the specklereduction element 40. The intermediate image 22, as transferred orpassed through the speckle reduction element 40, is projected by theprojection subsystem 20. The Relay lens 18 has a relatively long workingdistance Wa (see FIG. 3) on the order of 150 mm and is a relatively slowlens, around f/6. In some embodiments, relay lens 18 is telecentric onthe side facing the light modulators 12.

FIG. 3 shows an embodiment where the relay lens 18 has an f# (i.e.,first f# 19 in FIG. 3) that is greater than (e.g., about twice) an f#(i.e., second f# 24 in FIG. 3) of the projection subsystem 20. Stateddifferently, the exiting angle of relay lens 18 is about half that ofthe acceptance angle of projection subsystem 20. In some embodiments ofthe present invention, the relay lens 18 has an f-number of f/6 orgreater, and the projection subsystem 20 has an f-number of f/3 orsmaller. As mentioned earlier, the relay lens 18 can have a long firstworking distance Wa that may be needed to accommodate beam combiningfrom different imaging channels. Alternatively, the projection subsystem20 has a second working distance Wb that can be substantially less thanthe first working distance Wa. While the projection subsystem 20 needsto accommodate the optics of the speckle reduction system (e.g., specklereduction element 40), and perhaps a field lens, the space required ismuch less compared to accommodating combiner 14. In some embodiments,the first working distance is 150 mm or greater, and the second workingdistance is 50 mm or smaller. Since the projection subsystem 20 isoptically faster, but can have a shorter working distance, it becomesmuch easier to design and fabricate than a lens that is both fast andrequires a long working distance as is common to current digital cinemaprojection lenses.

Likewise, the design of relay lens 18 is advantaged as it is working atsmall magnifications (1×-2×). While relay lens 18 still provides a longworking distance Wa, this lens is optically slower than the conventionaldigital cinema projection lenses, and thus relay lens 18 is also lessexpensive to fabricate and design. Further, as the optical combiner 14is now in a relatively slow portion of the optical path, the opticalcoatings therein become much less difficult to design and fabricate, asthey only need to combine over smaller angles. In the case of MEMsspatial light modulators that differentiate input and output light basedon angular differences between the two, the contrast ratio is enhanced.These f-number and working distance relationships optimize the effectiveimaging performance to provide low cost optical design simplicity,simpler optical coatings, as well as high image quality parameters ofhigh contrast ratio and speckle reduction by increasing the angulardiversity discussed below.

Returning to FIG. 3, relay lens 18 directs its cone of light to form anintermediate image 22 at an intermediate image plane 21 proximate to thespeckle reduction system, which, in this case, includes a specklereduction element 40, which is moved by an actuator 49. The specklereduction element 40 alters the intermediate image 22, or the lightpropagation from there, in a manner (e.g., by phase changes, angularchanges, or both) that reduces speckle visibility in the projected imageby temporally changing the coherent interference of the reflections withthe display surface 30.

A movement-generating system (e.g., actuator 49 shown in FIG. 4) can bea part of the speckle reduction system. The movement-generating systemprovides vibration, rotation, or other repeating or random movement tothe speckle reduction element 40 while the intermediate image passesthrough it in order to reduce speckle.

In one embodiment, the speckle reduction element 40 is an opticaldiffuser, such as a volume or a surface relief diffuser (e.g., aholographic diffuser). In such cases, the movement-generating systemcomprises an actuator 49 known in the art to cause in-plane motion ofthe diffuser back and forth, or rotationally in the plane of thediffuser, such that the diffuser remains at or near the intermediateimage plane 21. In other words, “in-plane motion” means, for example,motion in a direction perpendicular to the optical axis (in the exampleof FIG. 2, axis O) of the speckle reduction element 40.

The diffuser, when used as speckle reduction element 40, is known tochange the random phase of image light associated with each intermediateimage pixel 23 forming diffused image pixels 23′, as shown in FIG. 5 b.The actual speckle reduction from the movement-generating system in thisconfiguration is dependent on several factors, including the attributes(size, shape, and distribution) of the diffuser features (or structure),the characteristics (rate and range) of diffuser motion, the number ofprojection lens resolution elements (image pixels) on the displaysurface 30, and the characteristics of the scattering features on thescreen (display surface 30). As the phase structure of the image lightassociated with the intermediate image pixels is changed by the motionof the speckle reducing element (diffuser) 40, the incident phase andposition to the display surface 30 is changed slightly as well, therebychanging the interaction with the display surface micro-structure, andthe speckle interference in the reflected light. The viewer's effectiveeye resolution (determined by the viewer's distance from the screen andthe viewer's personal visual acuity (20-20, for example)) also impactspeckle visibility. The speckle visibility decreases as the number ofdiffuser features and image pixels on display surface 30 increase, butis ultimately limited to a non-zero value.

It is noted that while an optical diffuser can be used for the specklereduction element 40, it also impacts light propagation through thesystem 50 in several ways. As one example, placement of a diffuser at ornear intermediate image plane 21 essentially defines a new object, thediffused image 22′ shown in FIG. 5 b, within the system 50, as theangular extent (or NA) of the light is increased. The change in angularextent can be modeled as a convolution of the light diffusion profileand the incident light distribution (relay lens F# (19)). Of course, theeffective etendue that the projection lens needs to accommodate islikewise increased. As a second impact, the effective size of theintermediate image pixels 23 at the intermediate image plane 21 isincreased, as the diffuser features and diffuser motion, introduceblurring and contrast loss, creating diffused image pixels 23′. Whenthis new image object, diffused image 22′, with its diffused imagepixels 23′, is imaged to the display surface 30 by projection subsystem50, the projected image quality, relative to resolution (or MTF) isreduced.

Again considering FIG. 2, relay lens 18 comprises at least one relaylens element L1, and can include an optional field lens L2, which can belocated adjacent the speckle reduction element 40 along the optical axisO, without intervening lenses between it and the speckle reductionelement 40. In the case of FIG. 2, the field lens L2 is located betweenthe speckle reduction element 40 and the relay lens element L1, andfacilitates direction of the intermediate image 22 into the specklereduction element 40 in a manner that causes the diffused image lightfrom the resulting diffused image 22′ to enter an acceptance aperture ofthe projection subsystem 20. In other embodiments, the field lens L2 canbe located downstream of the speckle reduction element 40, while stillprior to projection subsystem 20. Output from the speckle reductionelement 40 is a phase-altered image, which is projected by theprojection subsystem 20 onto a display surface 30 (not shown in FIG. 2).

While the image size of the intermediate image 22 at intermediate imageplane 21 may be equal to, smaller, or larger than the area of the lightmodulator 12, in many cases it is desirable for the image area to beequal to or larger than the area of the modulator 12, as NA collected bythe projection lens 50 can be reduced. In this regard, the image formingsystem (e.g., light modulators 12 r, 12 b, 12 g and combiner 14) canproject a combined initial image towards the relay system (e.g., relaylens 18), the initial image having a first size corresponding to thesize of the light modulators. The relay system (e.g., relay lens 18) canthen form an intermediate image 22 having a second size greater than orequal to the first size.

In some embodiments of the present invention, the second size, i.e., thesize of the intermediate image 22 at intermediate image plane 21, ismagnified by the relay system (e.g., relay lens 18) to be consistentwith a motion picture film size, such as 16 mm, 35 mm, or 70 mm filmformats. These motion picture film sizes have diagonals of 13.73 mm,25.81 mm and 52.80 mm for the 16 mm, 35 mm and 70 mm film formats,respectively. Having an intermediate image 22 at these motion picturefilm sizes allows the use of a conventional film projection lens inprojection subsystem 20, such as a lens designed and used forconventional 16 mm, 35 mm, or 70 mm film projectors, thereby reducingcost and simplifying design.

In particular, film-based projection lenses, such as lenses sold bySchneider Kreuznach of Germany, are offered in a range of nearly 30different focal lengths from 24 to 100 mm in order to accommodate thevariety of screen distances and diagonals present in the motion pictureindustry. This variety of lenses allows theatre operators to select thebest solutions for their particular venues. Some lenses such as theVariable Prime lens are also designed to handle different film formatratios 1:137 to 1:1.85, while others use anamorphic optics to deliverwide format Cinemascope content of 1:2.39 format. This wide availabilityand format flexibility offers a significant advantage over conventionaldigital cinema lenses that are more expensive and limiting. Schneideroffers only twelve fixed lenses for digital cinema. Perhaps moresignificantly, the f-number of these common film projection lensestypically range between f/1.7 to f/2.8. This lower f-number range isparticularly suited to reduce the speckle from the coherent lightsources by using temporally averaged angular diversity as opposed tomore common random phase walk of a common diffuser. However, theselenses only have back working distances in the range of 30-57 mm, whichcan be too small to accommodate digital projector attributes such asbeam combining from three colors. In utilizing conventional filmprojection lenses, the availability and ease of changing format size isalso increased. Common anamorphic lenses are also capable of being usedto switch to formats like Scope for exceptionally wide viewing.

One consideration in utilizing conventional film projection lenses fordigital cinema applications is that many of the higher qualityconventional film projection lenses are designed to compensate for filmcurvature or “buckle”, which is illustrated in FIG. 5 c. When the imagearea of the film 60 is illuminated, the film emulsion absorbs light inaccordance with the image content. The resulting heat causes the film,which is an elastic polymer sheet material, to buckle or bow out ofplane by some distance “d”. This effect is compounded by the unevenillumination and uneven heating of the image area, through the apertureplate 62, relative to the surrounding un-illuminated areas provided forthe sound track, perforations, and the framing bars. The illuminatedarea expands, while the fixed area does not, forcing the imaging area toshift in the optical axis direction, thereby introducing as much asd˜150-400 μm of film surface curvature, buckling towards theillumination source. Thus, with respect to the projection lens 20, thefilm 60 is now a curved object that is imaged to the screen 30.

The least expensive film projection lenses often are designed for a flatimage plane, thus causing defocus on the outer edges of the imaged area(assuming the projectionist sets focus to the image center). Theselenses, if their performance characteristics are suitable, can bedesirable for use with a digital spatial light modulator 12 where theimage plane remains flat during projection. On the other hand, the moreexpensive film projection lenses 20 that provide the best projectedimage quality, are also designed to optimally image a curved object (thefilm 60) having a film buckle depth d ˜100-200 μm sag over a ˜1 inchwide area, to compensate for the film buckle effects, e.g., the filmplane deviation from illumination system heating.

With respect to the present invention for digital projection, theunlikely combination of imaging planar (flat) light modulators 12 to anintermediate image plane 21 that is co-aligned to a curved object planeexpected by a film-type projection lens, can be accommodated in severalways. Considering FIG. 2, relay lens 18 is relatively slow as comparedto the projection subsystem 20. In the case of relay lens L1, it is lessimportant to correct for the small image plane sag of 100-200 μm, as thedepth of focus of lens L1 is greater. Depth of focus (DOF), is shown inslide 5 a, and is defined by:

${DOF} = {\pm \frac{\varnothing}{2*{\tan\left( {\arcsin\left( \frac{1}{2*f\#} \right)} \right)}}}$

where φ=the blur circle diameter. To be resolvable, the size of theintermediate image pixels 23 presented to the intermediate image plane21 would approximately equal the blur circle diameter, if not larger.For example, if the spatial light modulators 12 comprise arrays of 10 μmpixels that are imaged at a magnification of 1.2× to the intermediateimage plane 21 then the resulting 12 μm intermediate image pixels shouldbe comparable in size to the blur circle diameter (φ), or 2-3× larger.The depth of focus is a distance in the Z direction in which the size ofthe blurred spot grows by some defined amount that is deemed tolerable.Diffraction, aberrations, or defocus, or combinations thereof, invarying quantities, can cause the spot blurring. For example, Rayleigh'squarter wave criterion is a common metric used in imaging systems. Usingthe equation above, and given an f/6 relay lens 18, the depth of focusat the intermediate image plane 21 would be about 120 μm, which is onthe order of a standard film projection lens curvature. So in this case,the relay lens 18 roughly accommodates, within the depth of focus, theimage plane curvature without significant correction required.Nominally, the relay lens 18 is positioned axially along the opticalaxis O, such that the best quality image location (best MTF) provided bythe relay lens 18 substantially overlaps the best object conjugate planelocation of the projection lens 20. Relay lens 18 can also be designedto present a curved image of the modulators 12, with an appropriatecurvature, to the film type projection lens 20.

On the other hand, a mis-match between a projection lens 20 designed tooptimally image a curved object and a planar intermediated image cannotbe readily left uncorrected, as the depth of focus (DOF) of theseprojection lenses is comparatively small. For example, using anintermediate image pixel size of 12 μm and a projection subsystem 20 off/1.7, the depth of focus is only about 36 μm. This small depth of focusjustifies the need to match the curved plane of best imaging of a filmtype projection lens 20 with the intermediate image 22 and the surfaceof the speckle reduction element 40.

In one embodiment shown in FIG. 4, the speckle reduction element 40 hasa curved surface 43 through which the intermediate image 22 is received.In other embodiments (not shown), the speckle reduction element 40 has acurved surface through which a diffused image 22′ or intermediate image22 exits, to be imaged by projection lens 20. In either case, thecurvature can be convex or concave. Regardless, the curved surface has acurvature that matches or substantially matches the film bucklecurvature for which these commercially available film type projectionlenses are corrected. Surface structures can be place onto curvedsurface 43 in a random or ordered pattern. In one embodiment lensletsare formed ontop of the general curvature. In an alternate embodimentthe surface structure can be of phase depth by etching, polishing, ormolding. In cases where the curved surface 43 of the speckle reductionelement 40 is fabricated using etched or polishing processes, devicefabrication can be relatively straightforward. By comparison, it can bedifficult to fabricate the surface features of a lithographically curvedspeckle reduction element 40 as this process works primarily with flatwafers. It would be possible to lithographically pattern the surfacefeatures of a speckle reduction element 40 on a flat substrate, and thencreate flexible or curved speckle reduction elements 40 by replicationand molding processes. For example, a flexible master could then be usedto cast the speckle reduction surface features onto a spherical surfaceplane base or a plano surface with a second surface that containsoptical power. Thus, an inexpensive replicated speckle reduction element40 can be created that has optical correction for a curved image plane.

An alternate method, in lieu of generating a curved surface on thespeckle reduction element 40 is to add a correction lens system (L2 inFIG. 2 could be configured as such a corrector lens) to correct for thispre-existing image curvature of commercially available film projectionlens. This correction lens system, separate from speckle reductionelement 40, would only need to be a simple corrector lens with verylittle optical power placed between the relay lens element L1 and theconventional projection subsystem 20. In this regard, the correctionlens system (e.g., L2) can be located upstream or downstream of thespeckle reduction element 40, and can be located adjacent the specklereduction element 40 in the optical axis O with no intervening lensesbetween it and the speckle reduction element 40. In addition, thecorrection lens system can comprise a lens having only one or both sidescurved, and such curvature can be convex or concave, depending upondesign choice. Or, as with all lens systems, more than one lens may beused. Accordingly, one skilled in the art will appreciate that theinvention is not limited to any particular implementation of thecorrection lens system, so long as it compensates for the film bucklecorrection of the standard cinema film type projection lenses discussedpreviously. Further, this correction lens system could be standard oroptional for projection systems 20 depending on the particularcommercially available lens selected.

Turning back to a general discussion regarding speckle reduction element40, such element 40, regardless of film buckle corrections andindependent of the type of relay and projection lenses used, is locatedat, or substantially at, the intermediate image plane 21, according tosome embodiments of the present invention. Further, regardless of filmbuckle corrections and independent of the type of relay and projectionlenses used, the speckle reduction element 40 can be diffusive, such asby including a diffuser, or refractive, such as by including a lensletarrangement 44 thereon, as will be discussed below.

In embodiments where the speckle reduction element 40 is diffusive, suchelement 40 provides phase shifting and diffusion of the intermediateimage 22. As discussed above, courtesy of actuator 49, this shiftingoccurs on a temporal basis, such that the effective spatial coherence ofthe light output by the projection subsystem 20 is averaged by the eyeto effectively reduce the speckle perceived by a viewer. The specklereduction element 40 can be a diffractive element that can be fabricatedfrom many different materials such as glass, fused silica, plastics orepoxy. For high-light-level-polarization-based optical systems it isimportant that the material does not absorb light such that heat inducedstress birefringence occurs. Similarly, a variety of methods can be usedto fabricate speckle reduction element 40, such as etching, polishing,molding, lithography, and holography. Again, for polarization sensitivesystems, a method that does not induce stress birefringence is desired.These diffusers may be created with random or periodic patterns tominimize speckle.

As mentioned previously, with respect to FIGS. 5 a and 5 b, use ofdiffusers can cause image blurring and loss of resolution. Additionally,the increased angular spread introduced by conventional diffusers, whilebeneficial to reducing the speckle effect, also increases the effectivesystem Lagrange, and thus the required cone angle (or numerical aperture(NA)) to collect all of the diffused light. This decrease in the f# ofthe projection lenses 20 can increase the cost and difficulty of lensfabrication. For example, common ground glass diffusers behaveessentially in a Lambertian manner, where significant amounts of light55 spill outside the collection f-number 24 of the projection subsystem20 as shown in FIG. 3. Even modern holographic diffusers cansignificantly increase the angular extent, increasing spill light 55,light efficiency loss, projection lens aperture, and image blurring.

Light diffusion or scattering occurs from a combination of refractiveand diffractive effects, which can be volumetric or surface related.Diffraction from the surface of speckle reduction element 40 causes anangular spread containing lower order (angle) and higher order (angle)content. It is commonly known that for diffraction from a circularaperture, roughly 99% of the energy from diffracted light falls withinthe 4^(th) dark ring of the Airy disk pattern formed by the aperture. Itcan be desirable to design the diffusive structure such that most of theangular spread by diffraction is within the acceptance aperture (F# 24)of the projection subsystem 20. For example, the speckle reductionelement 40, in some embodiments where such element 40 is significantlydiffractive, directs the energy from the 4^(th) order and below into theacceptance aperture of the projection subsystem 20. There is adiminishing return in energy collection in requiring a lower f-numberprojection lens.

While a diffusive speckle reduction element 40 is effective at reducingperceived speckle, the surface treatments and structure types thatcreate a diffused image 22′ from the intermediate image 22 often createa loss of energy due to diffraction that can overfill the projectionsystem 20. This diffused image 22′ essentially becomes an objectrelative to the projection subsystem 20, with new wavefront and imagequality parameters. While aberrations from relay lens 18 are imparted tothe pixel structure, size, and shape, of the diffused image pixels 23′of diffused image 22′, the projection subsystem 20 cannot be optimizedto correct for them because the original phase (wavefront) content islost by the diffusive speckle reduction element 40.

An alternative method to speckle reduction to that of using a diffusivespeckle reduction element 40 is to use a refractive speckle reductionelement 40 that angularly shifts the intermediate image 22 whilepreserving at least some of the original phase content. As before, therefractive speckle reduction element 40 can be placed at theintermediate image plane 21 where the intermediate image 22 is formed bythe relay system (relay lens 18) and other upstream optics. Unlike thediffused image 22′ generated by a diffusive speckle reduction element40, a refractive, speckle reduction element 40 placed at orsubstantially at intermediate image plane 21 passes intermediate image22 while preserving substantial wavefront data projected by relay lens18.

In some embodiments, this refractive speckle reduction element 40 is astructured window element 45 comprising a lenslet arrangement (44 inFIGS. 3, 6 a, and 6 b) that is temporally moved as was described withrespect to a diffusive speckle reduction element 40. Unlike a diffusivespeckle reduction element 40 that creates random phase walk, such astructured window element generates temporally varying angular diversitythat changes the interaction of the incident light with themicrostructure of display surface 30, and thus the reflected lightinterference, thereby reducing speckle.

As shown in FIG. 6 a, the structured window element 45 can beconstructed of lenslets 41, each with an aperture A, formed on asubstrate 46. Consequently, as illustrated in FIG. 6 b, each lenslet 41acts like a sub-aperture field lens that deflects the image lightcollected by the projection lens 20 about the acceptance aperture of theprojection lens as the structured window element 50 is moved by actuator49. A lenslet 41 samples at least one intermediate image pixel 23 formedby the relay lens 18 and then redirects the angular extent (cone orsolid angle 25) of the light associated with a given intermediate imagepixel 23 into different portions (deflected solid angle 25′) of therelatively large acceptance aperture (captured solid angle 26) of theprojection subsystem 20 on a temporal varying basis, in conjunction withmovement of such structured window element 45 by the movement generatingsystem (e.g., actuator 49). Consequently, the movement generating systemsteers image light of the intermediate image 22 into an acceptanceaperture of the projection subsystem 20 as the intermediate image 22 istransferred through the arrangement of lenslets 41 of the specklereduction element. The projection subsystem 20 then projects the steeredimage light of the intermediate image, as transferred through the movingspeckle reduction element onto a display screen, which reflects areduced-speckle image. Because the lenslets 41 primarily provide amechanism for beam steering, rather than diffusion, for a given fieldpoint (a given intermediate image pixel 23), the corresponding on-screenimage is formed in substantially the same location on display surface30, whether the image light is deflected upwards or downwards, or to theside, or minimally, or not at all, depending on the position of thenearest lenslet to that given pixel at a given point in time. Thus,through the combination of each moving lenslet 41 and projectionsubsystem 20, angular steering occurs that is delivered to the imageprojection surface 30 (shown in FIG. 1) with minimal image quality loss.Any image quality loss would be due, at least in part, to the smallchange in optical power that each lenslet 41 delivers. Residualdiffusion effects may also increase the collected solid angle from anintermediate image pixel 23, or reduce image quality.

As mentioned earlier, a refractive speckle reduction element 40 can be astructured window element 45 having lenslets 41 that each have anaperture A, as shown in FIG. 6 a. The size of that aperture isrelatively large compared to the surface structures of common diffusersurfaces like etched glass. While the lenslet aperture A may be round,square, or other shape, it is useful to understand the properties thatdeliver proper sampling of the pixel size to reduce speckle and delivermost of the light to the display surface 30. The diffraction equationfor a circular aperture shows the relationship between the lensletdiameter (aperture A) and the angular spread of the 4^(th) dark ring isgiven by:

A=4.24*λ/α

-   -   Where:        -   A=lenslet diameter or aperture        -   α=angular spread (radians)        -   λ=wavelength of interest

For example, if lens 20 a is an f/2.8 lens, it has an acceptance angle αof approximately 0.18 radians. For a wavelength of 0.000550 mm light,the smallest lenslet size (diameter A) is approximately 13 μm. This sizeis on the order of the spatial light modulator pixels (5-15 g/m), or theimages thereof (i.e., the intermediate image pixels). At substantiallysmaller sizes than approximately 13 μm, significant amounts of light arediffracted or scattered and are lost. Therefore, a solution has eachlenslet sampling one or more intermediate image pixels 23 of theintermediate image 22 at the intermediate image plane 21. In otherwords, each of all or substantially all of the lenslet apertures A isgreater than or equal to a size of an intermediate image pixel 23 at theintermediate image plane 21. In some embodiments, it can be beneficialto have lenslet apertures A greater than a size of N² intermediate imagepixels of the intermediate image at the intermediate image plane 21. Insome embodiments, N is ˜2-4. Since the lenslet apertures A arerelatively large with respect to the size of an intermediate image pixel23, the pixels 23 essentially see the lenslet 41 effectively as awindow.

It is also useful to understand the parameters of the gaps 42, if any,between the lenslet structures. In this regard, a slit aperturediffractive model may be used where the lenslets 41 are directly abuttedto provide a simplified analysis. Slit apertures behave in a similarfashion as circular apertures in this regard. 83.8% of the energy occursinside the first dark ring, in this example about 4 μm, as given by:

Ag=1.22*λ/α

The percentage of energy inside the particular bright bands appearsbelow:

Order Circular Aperture Slit Aperture 0 order 83.8% 90.3% 1^(st) order7.2% 4.7% 2^(nd) order 2.8% 1.7% 3^(rd) order 1.5% 0.8% 4^(th) order1.0% 0.5%

There is a diminishing return in energy collection either in requiring alower f# projection lens 20 or in making the diffusive structuressmaller, especially where some of the diffraction from the 0th orderbeam is lost. For visible light, it is desirable to have even the gapstructures on the order of a magnified pixel dimension of Ag ˜5-20 μm.Again, designs where the dimension of the structure capture the fullzeroeth order to the full 4^(th) order would effectively simplify theoptics and capture increased energy. In other words, in someembodiments, the lenslet arrangement 44 is configured at least to pass afourth order energy or lower of the intermediate image, as transferredthrough the speckle reduction element, into an acceptance aperture ofthe projection subsystem. Stated differently, in some embodiments, anacceptance aperture of a lens in the projection subsystem 20 captures afourth order energy or below of the intermediate image, as transferredthrough the speckle reduction element.

Considering FIG. 6 b from another perspective, the incident F# (19) tothe intermediate image plane 22 provided by the relay lens 18 issubstantially preserved when the image light transits a specklereduction element 40 that is structured window element 45. That is,aside from any residual diffusion, both Lagrange and wavefront (orphase) information are substantially preserved, whether the lighttraverses the window-like lenslet gaps 42 or is deflected by thelenslets 41 into the larger acceptance F# of the projection lens (24).Thus, a magnified image of a given intermediate image pixel 23 is thenprovided at the display surface 30, with a minimal loss of imagequality, whether the image light traversed a deflected or un-deflectedpath through the projection subsystem 20.

Embodiments that include a structured window element 45 as a refractivespeckle reduction element 40 can have the lenslet arrangement 44 on theupstream side of the element 40 (facing the relay lens 18, e.g.) or onthe downstream side of the element 40 (facing the projection subsystem20, e.g., as shown in FIG. 3). In addition, in embodiments where therefractive speckle reduction element 40 has a curved surface tocompensate for film buckle corrections in projection subsystem 20, thelenslet arrangement 44 can be on the curved surface or on a flat surfaceon an opposite side of the curved surface, if applicable. The lensletarrangement 44 primarily causes the corresponding surface of thestructured window element 45 to be refractive. In some embodiments, thelenslet arrangement 44 entirely or almost entirely includes abuttinglenslets, as shown, for example, in FIG. 3. In other embodiments, suchas illustrated by FIGS. 6 a and 6 b, the lenslet arrangement 44 entirelyor almost entirely includes sparsely distributed (e.g., non-abutting)lenslets 41 with gaps 42 between lenslets 41. The gaps 42 comprise thespace between the lenslets 42, where the front and back surfaces of thesubstrate 42 are nominally parallel to each other. In either case, somediffusion is caused by the lenslet arrangement 44. For example, when thelenslets 41 are abutting, the valleys between abutting lenslets 41 causesome diffusion, by residual diffraction and edge effects. On the otherhand, when the lenslets 41 are not abutting, some diffusion can becaused by the spaces or gaps 42 between the lenslets 41 (by diffractionor scattering). Regardless of the cause of such diffusion, an acceptanceaperture (indicated by captured solid angle 26) of a lens in theprojection subsystem 20 captures such diffusion caused by spacingsbetween the lenslets or valleys between abutting lenslets 41, in someembodiments of the present invention. In embodiments where non-abuttinglenslets 41 are used, it can be desirable to have or substantially havethe gaps 42 between the lenslets 41 be large enough to allow diffusedlight from the lenslet gaps 42 to pass into an acceptance aperture ofthe projection lens 20. In one embodiment the lenslet gaps 42 aregreater than or equal to a size of a pixel of the intermediate image atthe intermediate image plane. Basically, as discussed previously, thismeans that the lenslet gaps 42 should be large enough (Ag) that they donot cause significant diffraction effects.

Further in this regard, in some embodiments, a combination element canbe used as speckle reduction element 40, the combination element causinga more equal amount of diffusion and refraction, or diffraction andrefraction, to generate both temporally varying angular diversity andsome level of random phase walk. For example, the lenslet gaps 42 maynot be planar, but have some mild surface structure, with a randomlymottled profile with large spatial features (several microns) andminimal sag, to introduce mild diffusion. In some embodiments, a lensletor prism array can be utilized where the surfaces of the lenslet orprism structure are further structured with variable or fixed phaseshifted depth content in addition to the angular shifts due torefraction. Thus, as the speckle reduction element 40 is moved, there isan angular shift of the image into the projection lens and also a phasewalk of the coherent light.

Embodiments that include a structured window element 45 having lensletarrangement 44 as a refractive speckle reduction element 40, thearrangement of the individual lenslets 41 can be or substantially behexagonal, diagonal, random, or linear in either of the two dimensionsto create the pattern. It can be beneficial in some embodiments to havethe movement generating system move the speckle reduction element 40in-plane a distance that is greater than or equal to a period of lensletrepetition (one lenslet and one gap (if present), e.g., vertical lensletperiod 38 horizontal lenslet period 39 in FIG. 6 a), thus allowing thefull range of angular diversity from a lenslet 41. This motion allowsaveraging over the temporal response of the eye. As with the priordiscussions regarding a diffusive speckle reduction element 40, thestructured window element 45 can be moved in plane (with the X-Y planeshown in FIG. 5 a) any number of ways, such as by a linear transducer orby being rotated using a motor. Consequently, one skilled in the artwill appreciate that actuator 49 can take any of a number of forms,including a piezoelectric translator, for example.

It can be beneficial in some embodiments to have out-of-plane motion(along the Z-axis (see FIG. 5 a)) of the speckle reduction element 40,regardless of whether it is diffractive or refractive, provided thatsuch motion is within the depth of focus of the faster projectionsubsystem 20, plus some margin for allowable defocus at the screen.Since relay lens 18 has a much larger depth of focus, which is nominallyoverlapped with the projection lens depth of focus, a system 50 designedunder this constraint will have little image quality loss. “Out of planemotion” in this context refers to motion that is parallel orsubstantially parallel to a direction of an optical axis of the specklereduction element 40, which, in the example of FIG. 2, is axis O. Suchout-of-plane motion that generally remains within the depth of focus ofthe projection subsystem 20 and relay lens 18 further reduces speckle byinducing additional angular diversity or phase shift withoutsubstantially impacting pixel resolution. In embodiments whereout-of-plane motion is provided to the speckle reduction element 40,in-plane motion can also be provided.

In order to reduce the loss of light from speckle reduction element 40,whether diffractive or refractive, it can be beneficial that the specklereduction element 40 be designed such that the combination angle of theimage light (within f# 19) from the relay lens 18, plus the angulardiversion and diffusion formed by speckle reduction element 40 (see FIG.6 b for the case of a refractive speckle reduction element 40) be equalor substantially equal to the capturable solid angle 26 or acceptancef-number 24 of projection subsystem 20 (shown as item 24 in FIG. 3). Forexample, if the f# out of a refractive speckle reduction element 40 issubstantially greater than around f/5 with a projection lens f# ofaround f/2.5, speckle reduction would be reduced due to the lack ofangular diversity. Alternatively, if the f# out of a refractive specklereduction element 40 is smaller than that of the projection subsystem20, some light will not be collected by the projection subsystem 20 andoptical throughput will be decreased.

Notably, when illumination is from lasers, internal components ofcoherent light projection system 10 can have a low etendue, typically inthe range of about f/6. Such low etendue is advantaged from an opticaldesign perspective, allowing the use of smaller, slower, and lessexpensive lens elements and light modulators internal to each of thecolor channels.

The digital micromirror or DLP device works most effectively when itsmodulated light, the light reflected from its mirror elements, issubstantially telecentric, emerging substantially parallel to theoptical axis. Low-etendue light sources such as lasers are advantagedfor providing illumination in telecentric systems and are well-suitedfor providing DLP illumination sources.

Embodiments of the present invention described herein help to compensatefor speckle by correcting it within an intermediate image. Anintermediate image can be formed in a size or format that emulatesconventional film formats, enabling the use of off-the-shelf projectionlens designs for subsequent projection onto the display surface. Thus,various embodiments of the present invention allow the use of laser andrelated highly coherent sources, advantaged for brightness and spectralcharacteristics, with digital light modulators, without excessivespeckle.

The invention has been described in detail with particular reference tocertain embodiments thereof. It is to be understood, however, thatvariations and modifications can be effected that are within the scopeof the invention. For example, lens elements could be fabricated fromany suitable type of lens glass or other optical material. Lens mountingarrangements of various types can be provided. A variety of types oflaser light sources can be used, including laser arrays, for example.Any of a number of different types of light modulators can be used,including digital micromirrors, liquid crystal display (LCD) devices,electromechanical grating devices such as grating electromechanicalsystem (GEMS) devices and grating light valve (GLV) devices, or othertypes of pixellated array devices. While embodiments using three primarycolors (RGB) have been described, embodiments of the present inventioncan also be used where more or fewer than three light sources ormodulators are utilized. Further, the terms “system” and “subsystem”often is used in this description to acknowledge that, although certainembodiments illustrated herein have a particular arrangement of lensesor other components, one of ordinary skill in the optical arts willappreciate that such particular arrangements could be replaced by one ormore other arrangements to achieve the same functions described herein.For example, the relay system is often described herein as including arelay lens 18. Such lens 18 could be replaced by a plurality of lensesthat still form an intermediate image 22 at the intermediate image plane21. The same reasoning applies to the projection subsystem 20 and theother systems described herein. Further, the terms “system” and“subsystem” are also used in this description to acknowledge thatadditional conventional components not shown or described herein canalso be present. For example, the relay system 18 may include a relaylens L1, but it likely also includes lens-mounting hardware, certainoptical coatings, etc.

PARTS LIST  5 Object Plane  7 Displayed Image Plane 10 Coherent lightprojection system 12r, 12g, 12b Light modulator 14 Combiner 16r, 16g,16b Light source 18 Relay lens 19 Relay lens f# 20 Projection subsystem20a Lens of projection subsystem 20b Lens of projection subsystem 21Intermediate image plane 22 Intermediate image 22′ Diffused image 23Intermediate image pixels 23′ Diffused image pixels 24 Projectionsubsystem entrance f# 25 Solid angle 25′ Deflected solid angle 26Captured solid angle 30 Display surface (or screen) 38 Vertical lensletperiod 39 Horizontal lenslet period 40 Speckle reduction element 41Lenslet 42 Lenslet gaps 43 Curved diffuser surface 44 Lensletarrangement 45 Structured window element 46 Substrate 49 Actuator 50Coherent light projection system 55 Spilled light 60 Film 62 Apertureplate L1 Relay lens element L2 Field lens (or correction lens) O Opticalaxis Wa, Wb Working distances d Film buckle Ø Blur circle diameter ALenslet aperture Ag Lenslet gap aperture size

1. A coherent light projection system comprising: a coherent lightsource system configured at least to emit coherent light; an imageforming system configured at least to interact with the coherent lightin a manner consistent with image data; a relay system configured atleast to form an intermediate image at an intermediate image plane fromcoherent light output from the image forming system, the intermediateimage being an aerial real image; a speckle reduction element located ator substantially at the intermediate image plane, the speckle reductionelement having a curved surface configured to transfer the intermediateimage through it; a movement generating system configured at least tomove the speckle reduction element; and a projection subsystemconfigured at least to project the intermediate image, as transferredthrough the speckle reduction element.
 2. The system of claim 1, whereinthe projection subsystem is further configured to correct for filmbuckle effects, and wherein the curved surface is configured at least tocompensate for the projection subsystem's correction of film buckleeffects.
 3. The system of claim 1, wherein the curved surface is asurface through which the intermediate image is received by the specklereduction element.
 4. The system of claim 1, wherein the curved surfaceis a surface through which the intermediate image exits the specklereduction element.
 5. The system of claim 1, wherein the curved surfaceof the speckle reduction element is an etched or polished surface. 6.The system of claim 5, wherein the curved surface comprises randomly orsubstantially randomly distributed surface structures.
 7. The system ofclaim 1, wherein the image forming system comprises light modulators anda dichroic combiner that aligns a plurality of color channels from thecoherent light source system onto a common axis.
 8. The system of claim1, wherein the movement-generating system is configured at least tocause motion of the speckle reduction element in a direction parallel toan optical axis of the speckle reduction element.
 9. The system of claim8, wherein the motion parallel to the optical axis is within a depth offocus of the projection subsystem.
 10. The system of claim 8, whereinthe motion parallel to the optical axis is within a depth of focus ofthe relay system.
 11. The system of claim 8, wherein the movementgenerating system is further configured at least to cause motion of thespeckle reduction element in a direction perpendicular to the opticalaxis of the speckle reduction element.
 12. The system of claim 1,wherein the speckle reduction element comprises a lenslet arrangementformed on a surface of the speckle reduction element, the lensletarrangement comprising lenslets each having an aperture, wherein each ofall or substantially all of the lenslet apertures is greater than orequal to a size of a pixel of the intermediate image at the intermediateimage plane.
 13. The system of claim 12, wherein the lenslet arrangementcomprises a random or substantially random distribution of lenslets. 14.The system of claim 12, wherein the lenslet arrangement compriseslenslets in or substantially in a hexagonal, linear, or diagonalpattern.
 15. The system of claim 12, wherein the lenslet arrangemententirely or almost entirely comprises abutting lenslets.
 16. The systemof claim 12, wherein the lenslet arrangement entirely or almost entirelycomprises non-abutting or sparsely distributed lenslets.
 17. The systemof claim 12, wherein the movement-generating system is configured atleast to move the speckle reduction element in-plane a distance that isgreater than or equal to a period of lenslet repetition.
 18. The systemof claim 12, wherein the lenslet arrangement is configured at least topass a fourth order energy or lower of the intermediate image, astransferred through the speckle reduction element, into an acceptanceaperture of the projection subsystem.
 19. The system of claim 1, whereinthe image forming system is further configured to project an initialimage having a first size, and wherein the intermediate image has asecond size greater than or equal to the first size.
 20. The system ofclaim 19, wherein the second size is consistent with a 16 mm, 35 mm, or70 mm film format.
 21. The system of claim 19, wherein the projectionsubsystem is further configured to correct for film buckle effects. 22.A method of projecting light comprising: generating coherent light froma coherent light source system; forming an image with an image formingsystem at least by interacting with the coherent light in a mannerconsistent with image data; forming an intermediate image at anintermediate image plane at least from coherent light output from theimage forming system, the intermediate image being an aerial real imageformed with a relay system; transferring the intermediate image througha curved surface of a speckle reduction element located at orsubstantially at the intermediate image plane; moving the specklereduction element with a movement generating system while theintermediate image is transferred through the speckle reduction element;and projecting the intermediate image, as transferred through thespeckle reduction element with a projection subsystem.